As described, for example, by J. Dakin and B. Culshaw in “optical fiber sensors”, Artech House, Boston, 1997, the sensors distributed in the optical fiber which are based on the effect of Stimulated Brillouin Scattering (SBS), have gained in last years an important role in different field of application, as for example in the field of structure monitoring or in the field of environment monitoring. Such a success is due mostly to the unique possibility that these sensors offer to carry out measurements of the physical quantity of interest (temperature and/or strain), with continuity and over long distances. Such sensors therefore proves as indispensable instruments whenever one requires a high number of measurement points, which have to be spatially distributed over long distances. In such cases, the use of sensors based on the SBS effect allow using an only optical fiber as sensing element, which provides simultaneously an amount of information which is comparable with that obtainable using hundreds or thousands of point-like sensors. The advantages reside in the low cost, and in the immunity to the electromagnetic interferences of the optical fiber used as element that is sensible to the quantity of interest.
As described for example in G. P. Agrawal, “Non-linear fiber optics”, Academic Press, Boston, 1989, the SBS effect origins from the interaction, along the optical fiber, between two counter-propagating optical waves with carrier frequencies that differ of a quantity ν, and of an acoustic wave at ν frequency. The acoustic wave at issue is generated directly by the two optical waves, by means of a process of electrostriction connected to the nature of the material of which the optical fiber is constituted (silica glass). More precisely, the interferential pattern generated by the spatial superposition of the two optical waves along the fiber generates, within the same optical fiber, zones of high field intensity, which compress due to the electrostriction effect. The last consists in the strain of a dielectric immersed in an electrical field.
The alternation of zones of higher and lower density is able to activate the generation of a coherent acoustic wave, provided that a condition of resonance between the propagation speed of the acoustic waves in the fiber and the speed of the interferential pattern movement along the fiber is verified. Since this last speed depends essentially on the frequency shift between the two counter-propagating optical waves along the fiber, the intensity of the generated acoustic wave will depend indeed on such a ν shift. The value of the frequency shift between the two optical signals for which one has the maximum intensity of the generated acoustic wave is called “Brillouin frequency shift”. The magnitude of such a shift is linked, as mentioned above, to the speed with which the acoustic waves propagate along the medium (the optical fiber in our case of interest), and is therefore a characteristic parameter of the fiber. Once supposed that the resonance condition of the frequency shift between the two optical signals is satisfied, the acoustic wave so generated acts as a “scattering” element with respect to the optical radiation at higher frequency, which is called pump radiation. The scattering of the pump wave, caused by the acoustic wave generated by electrostriction (the Brillouin stimulated scattering), produces a transfer of power from the pump wave the to the optical wave at lower frequency, that is called probe wave.
Therefore, after all, the stimulated Brillouin scattering will result in a transfer of energy from the pump wave to the probe wave, which will be all the stronger the closer the frequency shift between the pump wave and the probe wave is to the Brillouin shift of the optical fiber. Since the Brillouin shift depends on the elastic characteristics of the material (density and elasticity modulus), it will depend on the temperature conditions and strain to which the same fiber is subjected. This dependency enables to use the measurement systems of the Brillouin shift within temperature and strain sensors.
The use of suitable interrogation techniques allows effectuating the distributed measurements, i.e. to spatially resolve along the fiber the whole Brillouin shift distribution. The most common technique is based on the use of a pulsed pump signal, and of a continuous wave (CW) probe signal. The principle is similar to that of reflectometry in the time domain: one acquires in the time domain the probe signal received by one end of the fiber, starting from the launching instant of the pump impulse, the last being launched from the same end. In such a way, being known the speed of propagation of the pump impulse along the fiber, it is possible to correlate the arrival instant of the probe signal with the position of the fiber in which that signal has interacted with the pump impulse. By measuring the probe signal amplification as a function of the time and frequency shift between the pump wave and the probe wave, it is possible to derive the Brillouin shift value for each position along the fiber. The spatial resolution of the measurement depends essentially on the time duration of the used pump impulse: the longer is the impulse, the smaller will be the resolution with which the Brillouin shift along the measurement fiber is measured.
The poor signal-to-noise ratio associated to such measurement type, which essentially depends on the weak coupling between the pump light and the probe light, has driven towards the introduction of alternative detection techniques. A detection method which is very efficient is the one that operates in the frequency domain, and is described for example in D. Garus, T. Gogolla, K. Krebber, F. Schliep, “Brillouin optical-fiber frequency-domain analysis for distributed temperature and strain measurements”, J. Lightwave Technol., 15, 654-662 (1997). In such a type of interrogation technique, called BOFDA (Brillouin Optical Frequency-Domain Analysis), the measurement of the Brillouin shift is effectuated by launching in the fiber a continuous wave (CW) probe signal on one side, and a pump signal on the other side whose intensity is constituted by a CW component with a superposed sinusoidal component at a certain frequency. This technique comprises the measurement of the sinusoidal component at the same frequency, induced on the probe signal coming out from the fiber. Such a component must be measured in phase and modulus, with varying modulation frequency. In the end, what is measured is a complex transfer function, which represents the SBS interaction in the fiber between the pump light and the probe light, for a certain value of the frequency shift between the optical carriers of these two signals. In first approximation, such as transfer function can be correlated to the impulsive response of the system by a simple Fourier transform relationship. Therefore, once the data in the frequency domain are acquired, the last can be anti-transformed and then elaborated so as to derive the Brillouin shift distribution along the fiber.
A possible measurement setup which implements the BOFDA technique is represented in FIG. 1. As clear from the figure, the pump signal is intensity modulated at the frequency f by means of the optical modulator of intensity IM. The acquisition of the AC component at frequency f , which is present on the probe signal at the output of the fiber, is carried out by means of a vectorial network analyzer, which uses the intensity of the modulated pump signal as a reference. The optical/electric conversion of the signals is carried out by means of two photodiodes PD1 and PD2. On the basis of these two signals, the analyzer calculates modulus and the phase of the transfer function at modulation frequency f. Once carried out the measurement of the complex transfer function for a suitable range of frequencies f, such measurement is repeated several time with varying shift between the optical carrier of the pump signal and the optical carrier of the probe signal, so as to obtain a certain number of transfer functions suitable for the reconstruction of the Brillouin shift distribution along the measurement fiber.
The main advantage offered by the BOFDA technique, with respect to the traditional measurement techniques which operate in the time domain, consists in the fact that the measurements are carried out in the synchronous modality, and this will result in a better signal-to-noise ratio. However, the highest spatial resolution which is obtainable by means of the BOFDA technique is connected to the bandwidth with which the measurements in the frequency domain are acquired. By virtue of the necessity to obtain an always higher spatial resolution, it is appropriated to have the possibility of operating with the most possible large bandwidth. For example, the spatial resolution of a meter requires a functioning bandwidth, both of the network analyzer used for the calculation of the transfer function and the photodiodes used for the optical/electrical conversion of the signals, of around 100 MHz. The use of wide-band instrumentation, on one hand, will result in a higher cost of the apparatus which implements the measurement technique, and, on the other hand, the use of large-band photodiodes entails a higher quantity of noise in the measured data.